The invention relates to a method of manufacturing a valve sleeve which is used to form a hydraulic rotary valve.
A hydraulic rotary valve which may be used in a power steering apparatus, for example, comprises a valve rotor having a plurality of axial grooves formed therein and a valve sleeve rotatably fitted around the periphery of the valve rotor and having an internal surface in which axial grooves are formed which are capable of being brought into overlapping relationship with the opposite circumferential sides of the grooves in the valve rotor. A relative rotational displacement between the valve rotor and sleeve controls the supply and drainage of a hydraulic fluid to or from a power cylinder. Grooves in the valve sleeve are formed as blind grooves, namely, recesses are formed in an intermediate section of the sleeve to define the grooves while the both end sections of the sleeve must be left free from grooves.
A method of forming such a blind groove in the internal surface of a cylindrical valve sleeve is disclosed in U.S. Pat. No. 3,591,139. The disclosed method comprises repeatedly driving a cutter of a given size back and forth through a cylindrical member along an arcuate path to form a single groove, and synchronously rotating the member to form a plurality of grooves in a successive manner. Disadvantageously, this operation requires an equipment of complex construction and an increased length of working time.